Method for manufacturing titanium or titanium alloy thin oxide film having micro-holes

ABSTRACT

The cathode member 4 and the anode member 5 are respectively connected to a negative electrode and a positive electrode of a direct current power supply 3. In the processing device 1, the anode member 5 is a member to be processed, and a member using a thin film made of titanium or a titanium alloy is used therefor. A sulfuric acid solution or an electrolytic sulfuric acid solution S having an oxidizing agent concentration of 5 g/L or more with a hydrofluoric compound dissolved therein at 0.5% by weight or less is accommodated in the processing tank 2, and electrolysis processing is performed at a current density of 1-20 A/dm2. The processing device is applicable to a method for manufacturing, from a titanium or titanium alloy thin film, a thin oxide film of titanium or a titanium alloy having holes of 100 nm or less.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes, particularly to a method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes, which is suitable for forming micro-holes of 100 nm or less.

Related Art

A titanium or titanium alloy member is widely used because it is a light metal having high hardness and strength and having excellent characteristics of high corrosion resistance and high ductility. Furthermore, by applying anodizing processing to the titanium and titanium alloy members to form an anodized film, applications such as medical use and the like are expanded because not only the coloring, the improvement of abrasion resistance, and the function of the photocatalyst are exhibited but the ecological compatibility is also high.

As for the method for forming the anodizing film on the titanium or titanium alloy, it is known that, for example, electrolysis processing is performed in an electrolyte such as a mixed solution of sulfuric acid, phosphoric acid, and hydrogen peroxide with titanium or a titanium alloy as an anode (for example, patent literature 1, patent literature 2, and the like). According to these literatures, by applying a high voltage of 100 V or more to perform spark discharge processing to form micro-holes, and thereby a photocatalytic function can be imparted, and harmful substances such as organic compounds or bacteria can be removed.

LITERATURE OF RELATED ART Patent Literature

Patent literature 1: Japanese Patent Publication No. 6-41640

Patent literature 2: Japanese Patent Publication No. 8-984

SUMMARY Problems to be Solved

However, when the holes are formed in the anodizing film of the titanium or titanium alloy by the spark discharge processing in which a high voltage is applied, the formed holes are as large as about 1 μm. If the holes formed in the anodizing film are large as described above, there is a problem that when the titanium oxide film is bonded to a substrate such as a human tissue, the tissue is not densely filled and is difficult to be fixed. In addition, there is a problem that there is a technology that can make micro-holes smaller than 1 μm, but the film is detached during penetration and the shape as a film cannot be maintained because the holes have a tube shape.

The present invention has been made in view of the above problems, and an objective thereof is to provide a method for manufacturing a titanium or titanium alloy thin oxide film having holes of 100 nm or less from a titanium or titanium alloy thin film.

Means to Solve Problems

In order to achieve the above objective, the present invention provides a method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes, which takes a titanium or titanium alloy thin film as an anode, and performs an electrolysis processing at a current density of 1-20 A/dm² in a sulfuric acid solution with a hydrofluoric compound dissolved therein at 0.5% by weight or less or an electrolytic sulfuric acid solution having an oxidizing agent concentration of 5 g/L or more with a hydrofluoric compound dissolved therein at 0.5% by weight or less (invention 1).

According to the invention (invention 1), due to the etching effect of the electrolysis processing using a hydrofluoric compound, the holes of 100 nm or less, especially 50 nm or less can be formed in the titanium or titanium alloy thin oxide film.

In the above invention (invention 1), the sulfuric acid solution or the electrolytic sulfuric acid solution preferably has a sulfuric acid concentration of 10% by weight or more (invention 2).

According to the invention (invention 2), the holes of 100 nm or less can be formed in a short time in the titanium or titanium alloy thin oxide film.

In the above inventions (inventions 1 and 2), the hydrofluoric compound is preferably ammonium fluoride (invention 3).

According to the invention (invention 3), the holes of 100 nm or less can be easily formed in the titanium or titanium alloy thin oxide film.

In the above inventions (inventions 1 to 3), a processing time of the electrolysis processing performed using the sulfuric acid solution or the electrolytic sulfuric acid solution with a hydrofluoric compound dissolved therein is preferably 30-60 seconds (invention 4).

According to the invention (invention 4), the holes of 100 nm or less can be efficiently formed in the titanium or titanium alloy thin oxide film.

In the above inventions (inventions 1 to 4), preferably performing an electrolysis processing in an electrolytic sulfuric acid solution with no hydrofluoric compound dissolved therein after the electrolysis processing performed using the sulfuric acid solution or the electrolytic sulfuric acid solution with a hydrofluoric compound dissolved therein (invention 5).

According to the invention (invention 5), the second-stage electrolysis processing is performed in the electrolytic sulfuric acid solution with no hydrofluoric compound dissolved therein after the first-stage electrolysis processing is performed in the sulfuric acid solution or the electrolytic sulfuric acid solution with a hydrofluoric compound dissolved therein, and thereby the holes of 100 nm or less of the titanium or titanium alloy thin oxide film can be stabilized.

Effect

According to the method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes of the present invention, electrolysis processing is performed, taking a titanium or titanium alloy thin film as an anode, at a predetermined current density in a sulfuric acid solution or an electrolytic sulfuric acid solution with a hydrofluoric compound dissolved therein, and thereby a titanium or titanium alloy thin oxide film can be formed and holes of 100 nm or less, especially 50 nm or less can be formed due to the etching effect of the hydrofluoric compound and the oxidizing power of the electrolytic sulfuric acid solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a processing device to which a method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes of an embodiment of the present invention can be applied.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 conceptually shows a processing device to which a method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes of an embodiment of the present invention can be applied. In FIG. 1, a processing device 1 has a processing tank 2 and a cathode member 4 and an anode member 5 placed inside the processing tank 2; and the cathode member 4 and the anode member 5 are respectively connected to a negative electrode and a positive electrode of a direct current power supply 3. Besides, the processing tank 2 may be equipped with a constant temperature heater (not shown) for keeping the solution in the processing tank 2 at a desired temperature. In the processing device 1, the anode member 5 becomes a member to be processed, and a member using a thin film made of titanium or a titanium alloy is used therefor. In addition, the cathode member 4 is not particularly limited as long as it is an electrically conductive material, and a member made of titanium or a titanium alloy (including a thin film) can be used in terms of electrical conductivity, corrosion resistance, and the like.

In the first-stage electrolysis processing, a sulfuric acid solution or an electrolytic sulfuric acid solution S with a hydrofluoric compound dissolved therein is used as an electrolysis processing solution accommodated in the processing tank 2 of the processing device 1. Because there is no oxidizing power with only the sulfuric acid solution and the titanium or titanium alloy is only dissolved, the diameter of the micro-holes can be set to 100 nm or less, but it is difficult to set the diameter to 30 nm or less. In order to form finer holes, an electrolytic sulfuric acid solution having oxidizing power is used. The oxidizing agent concentration in the case of the electrolytic sulfuric acid solution may be any concentration that can be generated when sulfuric acid is electrolyzed to produce an electrolytic sulfuric acid solution, but the oxidizing rate becomes slow if the concentration is lower than 5 g/L, and the diameter of the micro-holes formed in the titanium or titanium alloy thin film used for the anode member 5 becomes large. Besides, the upper limit of the oxidizing agent concentration is not particularly limited, but the concentration of about 10 g/L is realistic in terms of the efficiency when sulfuric acid is electrolyzed to produce an electrolytic sulfuric acid solution.

The hydrofluoric compound may be a salt of hydrofluoric acid and basic substance, and ammonium fluoride is preferable in terms of good handleability and generality. For example, when the hydrofluoric compound is ammonium fluoride, the concentration thereof is 0.5% by weight or less. When the concentration of the ammonium fluoride exceeds 0.5% by weight, the dissolution of titanium proceeds excessively, the holes become large, and the holes have a tube shape. In addition, if the lower limit of the concentration of the ammonium fluoride is lower than 0.1% by weight, the dissolution of titanium does not proceed, the formation of micro-holes is not sufficient, the formed oxide film also becomes thin due to the lack of progress of the dissolution of titanium and cannot penetrate the titanium substrate, and thus it is not preferable. The concentration of the ammonium fluoride is preferably set to 0.25±0.05% by weight in terms of the formation of micro-holes in particular. Besides, even in the case of a hydrofluoric compound other than ammonium fluoride, the concentration thereof may be set to 0.5% by weight or less.

Furthermore, regarding the sulfuric acid concentration in the sulfuric acid solution or the electrolytic sulfuric acid solution S, if the sulfuric acid concentration is lower than 5% by weight, there are few H⁺(H₃O⁺) ions obtained by sulfuric acid electrolysis in the electrolysis processing described later, and thus the dissolution rate of titanium or a titanium alloy serving as the anode member 5 (member to be processed) becomes slow, and an oxide film is formed on the surface and stops the reaction. On the other hand, if the sulfuric acid concentration exceeds 50% by weight, the dissolution rate of titanium becomes too fast, the titanium is preferentially dissolved, and it is difficult to form a titanium oxide film. Therefore, the sulfuric acid concentration is preferably set to 5-50% by weight. Particularly, the sulfuric acid concentration is preferably set to 10-40% by weight.

Next, a method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes using the processing device 1 described above is described. First, if the cathode member 4 and the anode member 5 consisting of a member using the thin film made of titanium or a titanium alloy, which serve as the to-be-processed members connected to the direct current power supply 3, are suspended in the processing tank 2, the processing tank 2 is filled with the sulfuric acid solution or the electrolytic sulfuric acid solution S.

Then, a current is applied from the direct current power supply 3. Thereby, the titanium or titanium alloy thin film of the anode member 5 becomes an oxide film, and micro-holes are formed due to the etching effect of the hydrofluoric compound. If the current density applied here is lower than 1 A/dm², it is difficult to achieve stable control; on the other hand, the current density is too high if it is higher than 20 A/dm², and thus not only the diameter of the holes formed in the titanium or titanium alloy thin film becomes large, but the holes also become a tube. Accordingly, the current density is 1-20 A/dm², preferably 3-10 A/dm².

The temperature of the sulfuric acid solution or the electrolytic sulfuric acid solution S in the electrolysis processing is not particularly limited, but when the temperature of the sulfuric acid solution or the electrolytic sulfuric acid solution S is lower than 10° C., the effect of the oxidizing agent in the electrolytic sulfuric acid solution is not sufficiently exhibited, and an oxide film is not sufficiently formed. On the other hand, if the temperature exceeds 50° C., the oxidizing rate increases and the dissolution of titanium does not proceed because a passive oxide film is formed; holes are hardly formed, and it becomes difficult to penetrate the titanium or titanium alloy thin film serving as a substrate. Therefore, the above temperature range is not preferable. Accordingly, the preferable temperature of the sulfuric acid solution or the electrolytic sulfuric acid solution S is 10-50° C., particularly 20-50° C. In the processing tank 2, the temperature of the sulfuric acid solution or the electrolytic sulfuric acid solution S is preferably maintained at a predetermined temperature by a constant temperature heater (not shown) as necessary so that the temperature becomes the above-described temperature.

The time of the electrolysis processing as described above is not particularly limited, but the dissolution of titanium is insufficient and the micro-holes are hardly formed if the time is shorter than 30 seconds. Moreover, if the upper limit of the electrolysis processing time is too long, not only the dissolution of titanium proceeds and the formed holes are large and non-uniform, but the processing efficiency also decreases, and thus the electrolysis processing time is preferably shorter than 360 seconds, and particularly, the processing time is preferably about 40-60 seconds.

By the above electrolysis processing, a titanium or titanium alloy thin oxide film having micro-holes in which the micro-holes are formed is obtained as described below. That is, by the electrolysis processing performed using the sulfuric acid and the hydrofluoric compound in the sulfuric acid solution or the electrolytic sulfuric acid solution S, Ti³⁺ elutes at the anode and the micro-holes are formed.

Ti+3H⁺→Ti³⁺+3/2H₂  (1)

When electrolytic sulfuric acid is contained in the electrolytic sulfuric acid solution S, the following reaction also proceeds and TiO₂ is formed.

Ti+2H₂O→TiO₂+4H⁺+4e ⁻  (2)

The anodizing processing of titanium or a titanium alloy is a competitive reaction between the dissolution of Ti in formula (1) and the oxidation of Ti in formula (2), and it is possible to form micro-holes while forming an oxide film according to the Ti oxidation rate and the etching action of TiO₂ determined by the hydrofluoric compound. Then, if the concentration of the hydrofluoric compound is too high or the exposure time is long, the hydrofluoric compound penetrates into TiO₂, and the titanium inside is also dissolved. This increases the possibility of forming a tube-shaped weak oxide film, and thus careful consideration is desirably given to the sulfuric acid concentration and oxidizing agent concentration of the sulfuric acid solution or electrolytic sulfuric acid solution S, the hydrofluoric compound concentration, and the electrolysis processing conditions.

Furthermore, in the embodiment, preferably, after the above-described electrolysis processing (the first-stage electrolysis processing) of the sulfuric acid solution or the electrolytic sulfuric acid solution S and the hydrofluoric compound, the electrolysis processing (the second-stage electrolysis processing) performed using only the electrolytic sulfuric acid solution S with no hydrofluoric compound therein is performed.

The conditions for the second-stage electrolysis processing may be basically the same as in the first-stage electrolysis processing described above, and the processing time may be appropriately set according to the thickness of the titanium or titanium alloy thin film used in the anode member 5. By performing the second-stage electrolysis processing in the electrolytic sulfuric acid solution with no hydrofluoric compound dissolved therein, the holes of 100 nm or less of the titanium or titanium alloy thin oxide film can be stabilized.

As described above, the method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes of the present invention has been described based on the above embodiments, but the present invention is not limited to the above examples, and various modifications can be made. For example, the anode member 5 may be a titanium or titanium alloy thin film, or may have a shape in which the surface of an electrode material made of metal is covered with a titanium or titanium alloy thin film. In addition, because the diameter of the formed micro-holes varies depending on the hydrofluoric compound concentration, the sulfuric acid concentration, the current density, the temperature of the electrolytic sulfuric acid, and the time of the electrolysis processing, these conditions can be appropriately adjusted according to the desired micro-holes.

EXAMPLE

Hereinafter, the present invention is described more specifically with reference to Examples and Comparative Examples. However, the present invention is not limited by the description.

Example 1 and Comparative Example 1

A test piece of pure titanium of 100 mm×100 mm×0.04 mm (t) was prepared, and the cathode member 4 and the anode member 5 were formed using the test piece, thereby constituting the processing device 1 shown in FIG. 1. The anodizing processing was performed by the processing device 1 with the processing conditions for the first-stage electrolysis processing set as shown in Table 1. That is, the first-stage electrolysis processing was performed, for a processing time of 40 seconds, at a current density of 3.5 A/dm² and a temperature of 30° C. in an electrolytic sulfuric acid solution S, wherein the electrolytic sulfuric acid solution S is obtained by dissolving ammonium fluoride of 0.25% by weight in an electrolytic sulfuric acid solution with a sulfuric acid concentration of 10% by weight and an oxidizing agent concentration of 10 g/L. Next, the second-stage electrolysis processing was performed for 60 seconds in only the electrolytic sulfuric acid solution with the same concentration as in the first stage without dissolving ammonium fluoride.

The surface of the test piece of pure titanium of the anode member 5 after this processing was observed at a magnification of 100,000 times with a field emission scanning electron microscope (FE-SEM), it could be confirmed that a titanium oxide film having holes of about 20 nm was formed on the test piece of pure titanium of the anode member 5. Besides, for comparison, in the case (Comparative Example 1) of performing the processing for a processing time of 40 seconds at a current density of 3.5 A/dm² and a temperature of 30° C. in only the electrolytic sulfuric acid solution having the same concentration as in the first stage without dissolving ammonium fluoride, a titanium oxide film was formed, but no micro-holes were formed.

Examples 2-4 and Comparative Examples 2 and 3

Test pieces of pure titanium were processed in the same manner as in Example 1, except that the electrolysis processing conditions for the first stage were set as shown in Table 1. Besides, Table 1 also describes the processing conditions for Example 1 and Comparative example 1. The surface of the test pieces of pure titanium of the anode member 5 after the processing was observed at a magnification of 100,000 with a field emission scanning electron microscope (FE-SEM), and the results of measuring the diameter of the micro-holes of the titanium oxide film formed on the test piece of pure titanium of the anode members 5 are shown together with the results of Example 1 and Comparative Example 1 in Table 2.

TABLE 1 First-stage electrolysis processing conditions Sulfuric Oxidizing Ammonium acid agent fluoride Current Example concentration concentration concentration Temperature density Time No. (% by weight) (g/L) (% by weight) (° C.) (A/dm²) (second) Example 1 10 10 0.25 30 3.5  40 Example 2 10 10 0.25 10 3.5  60 Example 3 10 10 0.25 30 3.5 300 Example 4 10  0 0.25 30 3.5  40 Comparative 10 10 0   30 3.5  40 Example 1 Comparative 10 10 0.7  30 3.5  50 Example 2 Comparative 10 10 0.25 30 28    50 Example 3

TABLE 2 Example No. Diameter of micro-holes Example 1 About 20 nm Example 2 About 15 nm Example 3 About 40 nm Example 4 About 50-100 nm Comparative Example 1 No hole Comparative example 2 About 30 nm (tube shape) Comparative example 3 About 50 nm (tube shape)

As is clear from Tables 1 and 2, in Example 2 in which the temperature of the electrolytic sulfuric acid solution S in the first-stage electrolysis processing was 10° C., a titanium thin oxide film having micro-holes of about 15 nm could be manufactured. In addition, in Example 3 in which the processing time was as long as 300 seconds, a titanium thin oxide film having micro-holes of about 40 nm could be manufactured. Furthermore, in Example 4 in which the first-stage electrolysis processing was performed in only the sulfuric acid with ammonium fluoride dissolved therein, a titanium thin oxide film having relatively large holes could be manufactured. As can be known from these examples, the diameter of the micro-holes formed in the titanium thin oxide film can be adjusted by adjusting the conditions for electrolysis processing.

On the contrary, in Comparative Example 2 in which the electrolysis processing is performed in the electrolytic sulfuric acid solution with ammonium fluoride of 0.7% by weight dissolved therein and in Comparative Example 3 in which the current density was as high as 28 A/dm², the etching effect was too strong and the micro-holes had a tube shape, and the mechanical strength of the obtained thin film was low.

REFERENCE SIGNS LIST

-   -   1 processing device     -   2 processing tank     -   3 direct current power supply     -   4 cathode member     -   5 anode member     -   S sulfuric acid solution, electrolytic sulfuric acid solution 

1. A method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes, which takes a titanium or titanium alloy thin film as an anode, and performs an electrolysis processing at a current density of 1-20 A/dm² in a sulfuric acid solution with a hydrofluoric compound dissolved therein at 0.5% by weight or less or an electrolytic sulfuric acid solution having an oxidizing agent concentration of 5 g/L or more with a hydrofluoric compound dissolved therein at 0.5% by weight or less.
 2. The method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes according to claim 1, wherein the sulfuric acid concentration of the sulfuric acid solution or the electrolytic sulfuric acid solution is 10% by weight or more.
 3. The method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes according to claim 1, wherein the hydrofluoric compound is ammonium fluoride.
 4. The method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes according to claim 1, wherein a processing time of the electrolysis processing performed using the sulfuric acid solution or the electrolytic sulfuric acid solution with a hydrofluoric compound dissolved therein is 30-60 seconds.
 5. The method for manufacturing a titanium or titanium alloy thin oxide film having micro-holes according to claim 1, performing an electrolysis processing in an electrolytic sulfuric acid solution with no hydrofluoric compound dissolved therein after the electrolysis processing performed using the sulfuric acid solution or the electrolytic sulfuric acid solution with a hydrofluoric compound dissolved therein. 